Question: Which of these numbers is composite? ${17,\ 19,\ 51,\ 73,\ 79}$
A composite number is a number that has more than two factors (including 1 and itself). 17, 19, 73, and 79 each have only two factors. The factors of 51 are 1, 3, 17, and 51. Thus, 51 is the composite number.